Cheating on a significant other or spouse, as far as the United States goes, is the most evil thing a person can do. While most other moral issues fall into many shades of gray, infidelity is still very much black and white for so much of the world.
On the other hand, cheating is becoming more commonplace for men and women (very much so depending on how wide you want to draw the window of what you consider cheating).
Now while I don’t condone cheating, it is one of the most understandable behaviors on Earth if we look at it through the lens of our 6 human basic needs that drive all human behavior:
Your life coach.